The present invention relates to the monitoring and control of systems, in particular to the monitoring of the state of a system in response to an input.
Typically the monitoring of the state of a system, such as a mechanical, electrical, electronic system or combination thereof is achieved by including in the system sensors which are designed to measure and output the value of certain properties, such as operating parameters and variables of the system. The state of the system, and its response to inputs can then be seen in the output of the sensors. However, there are situations where the provision of sensors to monitor the system directly is disadvantageous. This is particularly true where the presence of the sensor can alter the system being monitored, or can degrade its performance, for instance by increasing the power consumption of the system, e.g. by requiring power itself, by introducing friction, or by disturbing the response of the system to an input. The presence of physical sensors also tends to add complexity to the system, and usually adds mass, and again there are situations where mass is critical and complexity needs to be reduced in order to increase the reliability of the system. Such a situation is, for instance, the provision of systems on orbital satellites. In that situation there is a general desire to reduce power consumption, to reduce mass and to reduce complexity, and so the provision of physical sensors on systems is undesirable.
An electro-mechanical example of such a system is found in the use of linear electric motors to drive valveless compressors, particularly for driving Stirling cycle coolers or pulse tube coolers, valved compressors, for instance domestic and industrial “Freon” type refrigerators, Gifford McMahon (GM) coolers and oil-free gas compressors, pumps in clean circulation systems, such as for medical purposes, or generators driven by Stirling engines. The linear electric motor is an example of a linear transducer, which combined with non-contacting bearings and seals, offers a number of advantages such as oil-free operation, which can eliminate problems associated with oil contamination, and wear-free operation, because the absence of contacting surfaces eliminates friction and thus maintenance is not required. Further, lifetimes and reliability can be very high. Linear motors can also be designed to have a very high efficiency, partly through the elimination of friction, but also because linear motors are capable of highly efficient part-load operation by varying the motor stroke at a constant frequency. This is not possible with rotary electric motors.
As mentioned above it is desirable to be able to monitor the state of the system, in this case the linear motor and compressor, and a particular problem in the use of such linear electro-mechanical transducers is the measurement and control of the stroke and offset of the transducer. By “offset” is meant the mean position of the moving part of the transducer. Thus, when such a transducer is in sinusoidal motion, its mean position is not easily defined or measured. In contrast, in a rotary machine the stroke and mean position are fixed by the geometry and physical connection of the drive mechanism, for instance the crank dimensions etc. In a linear electro-mechanical transducer, however, the stroke and offset are determined by the dynamics of the moving components, and generally show considerable variation with different operating conditions. This causes a number of problems. For instance, if the stroke and offset are not carefully controlled, damage can occur if the axial movement of the transducer exceeds the design range because it can result in unintended contact between different components of the system. Also, close control of the stroke and offset is needed to optimise machine efficiency, particularly at part load.
Currently the stroke and offset of a linear electro-mechanical transducer can be controlled by attaching a suitable displacement transducer to the moving part of the electro-mechanical transducer. Such a displacement transducer can, for instance, be based on the measurement of inductance changes caused by movement of an iron core within electrical coils in conjunction with the movement of the transducer being monitored. Capacitance-based monitors are also possible. However, such monitoring transducers, and their associated electronics add significantly to the size and cost of the device. The additional complexity also reduces reliability. Thus this is an example of a system in which it would be desirable to monitor the state of the system while avoiding direct measurement as far as possible.
U.S. Pat. No. 5,342,176 discloses a method of measuring the stroke of a compressor piston driven by a linear motor, on the basis of the voltage and current signals in the coils of the linear motor. With the valved system described in that patent the piston velocity is assumed to be proportional to the back EMF developed by the motor, and the piston stroke can be determined by integrating the piston velocity over time. The back EMF is deduced from the input voltage and current by assumptions based on a standard equivalent electrical circuit, including the motor inductance and resistance, both of which are assumed to be constant. It is also assumed that the force developed by the linear motor is proportional to the current in the motor, and is independent of position. However, the assumption that the force developed by the linear motor is directly proportional to the current, (the two being related by a constant known as the electro-mechanical transfer constant) is not correct for typical systems. Typically the electro-mechanical transfer constant depends on position. The described method for determining the average piston displacement also relies on having an expansion pressure for the piston that is equal to the fill pressure. That, also, is specific to the particular compressor design illustrated in the patent and is not generally applicable to other applications. It should also be said that if any of the system parameters of the compressor or linear motor vary, for example because of temperature variation or a fault condition, these cannot be taken into account and the measurements become less accurate or erroneous.
It is an object of the present invention to provide for the accurate monitoring and control of a system in an indirect way, and which reduces the need for sensors to measure the state of the system directly.
The present invention therefore provides a method and system for estimating the operating parameters and/or variables of a system which uses a parameterised model of the system and in which the model is fed with the same input as the system, the response of the model is compared with the response of the system, and the comparison is used to improve the parameters of the model. The values of the parameters and variables of the model can then be taken to be a good estimate of the values of the corresponding properties of the system itself. This technique can be incorporated into a feedback control loop, so that the input to the system is controlled on the basis of the model.
In more detail, the invention provides a method of monitoring the state of a system in response to an input to the system by providing an estimate of a value of at least one of a plurality of system properties including system variables and system parameters comprising the steps of:—
providing a parametrised model of the system in which respective model parameters and variables correspond to said plurality of system properties,
providing an input to said model corresponding to the value of an input to the system,
measuring one of said system variables,
comparing said measured one of said system variables with the corresponding model variable and outputting a measure of the difference between them,
optimising the model to reduce said difference,
generating a signal representative of an estimate of another one of said system properties based on the value of the model parameter or variable corresponding thereto.
The term “variable” is used here to mean properties of the system which can be measured such as current, voltage, position (and its derivatives), force, and functions of such variables and the corresponding values in the model. The term “parameter” is used to mean quantities which relate variables to each other, such as inductance, resistance, spring constants, the offset, and, in the embodiment below, the moving mass of the system. The distinction between variable and parameter, though, is to some extent arbitrary or dependent on the application and in general useful quantities will be functions of both variables and parameters.
The model may be optimised by iteratively comparing a measured system variable with the corresponding model variable and adjusting one or more of the model parameters, and determining from the optimised model an estimate of said at least one system property.
The optimisation need not continue until the agreement between the model and the system is maximum, but instead can continue for a preset number of iterations, or until the model and system are sufficiently close. The difference between them can be measured as the RMS difference or as the cross-correlation, and the optimisation process may proceed by a global optimisation algorithm which varies one or more of the model parameters.
In one embodiment of the invention the system comprises an electro-mechanical transducer, such as a linear motor, and the input may be the electrical voltage and the measured system variable be the electrical current in the transducer. However the motor may be driven by current input in which case the measured system variable is the electrical voltage, or alternatively the drive input may be the force, with the measured system variable being the voltage or current in the transducer. Linear motors can, of course, be used as generators and the invention is equally applicable to them.
Preferably the electro-mechanical transfer constant is a function of the transducer position. In one embodiment of the invention the transducer is designed specifically so that the electro-mechanical transfer constant varies with position in a known way.
The model parameters preferably comprise parameters describing the mechanical behaviour of the transducer, and its load, such as the moving mass, a damping coefficient, a spring rate coefficient, and the model variables include the offset representing the mean position of the transducer. The model also preferably includes the electrical characteristics of the transducer such as the effective circuit resistance, the effective circuit inductance and the effective circuit capacitance. The transducer may be driven with a repetitive or nearly repetitive waveform of known frequency, e.g. a sinusoidal waveform.
Where the transducer has a plurality of electrical circuits, the model may be based on separate circuit equivalents for each of the circuits, or the circuits may be lumped together as a single equivalent.
In a particular application of one embodiment of the invention, the transducer is a linear motor which is used to drive a compressor. Single compressor units are inherently unbalanced and thus generate vibration. To reduce the vibration an active balancer may be added to the system and the invention may fixer provide a way of calculating the control for the active balancer. In this case the motor analyser is used to determined the out of balance forces generated and a balancer model is used to calculate the required input to the balancer in order to balance the out of balance forces. The balancer model may be optimised in the same way as the transducer model mentioned above. Also, with balanced compressor pairs the invention can be used to adjust one of the compressor drives so as to attain better matching of compressor forces.
The invention also provides corresponding apparatus for monitoring and optionally for controlling, a system. Parts of the invention may be embodied in computer software and the invention extends to a computer program comprising program code means, and to a programmed computer system, for executing some or all of the steps of the invention.